Heretofore, when a solderless tank to header arrangement was used, the gaskets used therein either were formed in various non-planar shapes in order to help assure a positive seal, or were retained flat and clamped by flat header surfaces requiring considerable clamping force to compress the gasket over the sealing area. Examples of such solderless prior art are Fieni U.S. Pat. Nos. 3,583,478 and 3,628,603; Albers et al U.S. Pat. No. 3,027,142; Taylor U.S. Pat. No. 3,993,126; and Perry U.S. Pat. No. 3,792,729.